What is the right course of action?

You answered correctly by selecting that you must allow the red car to pass first and then you may proceed before the blue truck.

According to German traffic rules, the yellow diamond sign means you have priority on the main road. The sign below it shows a junction where your priority continues only straight or to the right, but not left.

The red car is coming from your right on the main road, so it has priority and you have to allow it to go first. The blue truck is waiting on a secondary road with a yield rule, so you have the right to go before the truck once the red car has passed.

Understanding how priority signs and junction layouts interact is crucial. This rule ensures a smooth flow of traffic and prevents collisions at intersections. In summary, you must let the vehicle on your right with priority pass first, then you may proceed before vehicles coming from a side road without priority.